PROJECT SUMMARY/ ABSTRACT Revision anterior cruciate ligament (ACL) reconstruction represents an infrequent but clinically important challenge in orthopaedic practice. It is commonly reported that the results of revision surgery remain inferior to primary ACL reconstructions. These poorer outcomes include worse patient based outcomes, increased knee laxity, higher graft failure rate, meniscal degeneration, and chondral lesions. A number of reasons for the poorer outcome rate have been proposed, including compromised tunnel location, pathology untreated during the primary reconstruction, and greater reliance on allografts for revisions. In order to evaluate the contributions of these and other factors to outcome, large numbers of patients who are undergoing revisions must be identified and followed prospectively. However, due to the relative infrequency of revision ACL reconstructions in any one center, a large multicenter study is necessary to accumulate enough subjects over a reasonable time frame to allow for multivariable analyses. With this in mind, the Multicenter ACL Revision Study (MARS) group was established as an 83 surgeon multicenter consortium to perform a prospective longitudinal cohort analysis of revision ACL reconstruction. This is a mixed group of academic and private practice physicians and has been supported and endorsed by the American Orthopedic Society for Sports Medicine (AOSSM). Our previous grant (5R01 AR060846) has allowed us to enroll over 1,200 patients and obtain both short (2- year) and mid-term (6-year) follow-up. This study design involves a longitudinal prospective cohort for whom we have baseline, 2-year and 6-year follow-up on. At both follow-up time points, we have been able to obtain a minimum of 80% follow-up using validated patient-reported outcome questionnaires and 92% via phone follow-up. The objective of this grant renewal is to assess the 10-year progression of outcomes following revision ACL reconstruction, and to determine how the initial factors at the time of revision surgery may influence and predict disease progression. New to this proposal is the addition of PROMIS patient reported outcomes to the entire cohort, as well as the addition of a nested cohort, for which we aim to bring back 400 of these enrolled patients for onsite standardized radiographs and physical exam measures. The focus of this renewal application is to characterize the 10-year natural history of this unique dataset, quantified by 3 complementary methodologies: patient-reported outcomes, radiologic and physical exam measures. This renewal will build upon the findings demonstrated at 2 and 6 years and determine if these continue to hold true at long-term follow-up.